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Insulating With Grout Filled Block (CMU) for a Higher R-Value

Updated on June 29, 2012

Before we can begin to talk about insulating a home by filling in standard concrete blocks with filler, we have to discuss the terminology for insulation. This terminology is called R-Value.

What Is R-Value

R-value is defined as a material’s resistance to heat flow. The higher the R-value of a material is the better it is at resisting heat flow which makes it a better insulator. While R-value is often discussed in terms of fiberglass insulation that has been added to a structure, the actual materials that comprise the structure have R-values of their own. Using a highly heat flow resistant material for your foundation and walls can significantly reduce your expenditure on heating and cooling in the finished building.


Grout Filled Block

For the purposes of clarity, and having a basic unit of comparison, we will use a concrete masonry unit (CMU) as our basic block. There are three sizes, 4-inch, 8-inch and 12-inch. The standard in the United States is the 8-inch variety so this is the one we will be using for our comparisons. The R-value of an unfilled 8-inch by 8-inch by 16-inch CMU is 1.11. This includes two 4-inch by 4-inch by 8-inch spaces on the interior of the block. The R-value of this ambient space is 1.00 (any space from ½-inch to 4-inches has a value of 1.00). This means that the R-value of the actual concrete block is a paltry 0.11.


A common misconception is that adding more cement in the center of the block will increase the R-value of the block; in fact the opposite is true. Concrete transfers heat faster than open air. A concrete filled block has an R-value of .19. The initial R-Value of 1.11 value and a space of 4 inches of fill to deal with, we can begin to calculate the value of the block when filled with various materials.


Fill Materials

There are several materials that can be used to fill concrete block. Some of the most popular fills are cellulose, cotton batts, denim batts, fiberglass (both loose-fill and batting), insulating foam, Phenolic foam spray insulation, Polyurethane foam and Silica Aerogel. Any of these materials can be used to fill a CMU.


The following list gives the R-value for each of these materials per inch. Keep in mind that the fill space is four inches so these R-values are multiplied by four.


Material R-Value


Cellulose 3.8


Cotton Batts 3.7


Denim Batts 3.7


Loose Fiberglass 3.7


Fiberglass Batts 4.3


Insulating Foam 3.9


Phenolic Foam Spray insulation 7.0


Polyurethane Foam 6.8


Silica Aerogel 10



To find out the R-value of your grout filled block, take the R-value of the material fill and multiply it by four then add 0.11 for the cement block. So, a Silica Aerogel filled block would have an R-value of 40.11 where a loose fiberglass filled block would come in at 14.91.


Savings per R-Value Point

There are several online calculators that can help you determine the savings made by installing a higher rated insulation. The standard rule is that for every 1 point of R-value that you increase there is a 1% savings in overall heating and cooling costs. The question then becomes, is a higher R-value worth the cost? This depends on the cost of heating and cooling your home. If you project a $37.00 a year savings and the cost of the insulation is $740, it will take 20 years to recoup your initial investment.


Green Building


With the recent movement toward a leaner, greener world, it makes sense that the topic of R-Value and house insulation would come into play. To be precise about R-Value, we have to understand that no insulation will prevent heat transfer -- it just slows it down. The amount that the transfer is slowed, directly affects the need to heat or cool the interior of the building. This means less energy is used for these processes.


When making a green choice in insulation, one of the best is a denim batt. These are usually made from recycled jeans and have the same R-value as traditional fiberglass. Not only is the material saved from becoming landfill, but it keeps your home insulated. Some other good green materials to fill a CMU with are cotton balls, shredded clothing (cotton based) or even straw. Each of these materials has an R-value higher than an empty block alone.



The R-value of a grout-filled block is determined by the material used to fill it. A strict, grout-filled block, is a poor insulator with a value of 0.19 whereas one with a foam or gel insert can range all the way up to 40.11. If you are interested in saving money in the long term, a high R-value is essential. This is also essential in an area where there are extremes in temperature. A good R-value will prevent either warm or cool air from leaking out and costing you more money.



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